1. Field of the Invention
The present invention relates to an intake air quantity measuring apparatus, and more particularly to an apparatus for measuring a quantity of the intake air which flows into an intake air duct of an internal combustion engine.
2. Description of the Related Art
For measuring a quantity of the intake air supplied to an internal combustion engine, various air quantity measuring devices have been employed. In the prior devices, an intake air temperature sensing element and a flow speed sensing element are disposed in an intake air duct with planes of the elements parallel to the flow of the intake air, as disclosed in Japanese Patent Laid-Open Publication No. 60-230019, for example. Both elements comprise heat sensing resistors which form a bridge circuit with fixed resistors. In parallel with the flow speed sensing element, there is provided a heating resistor which heats up the heat sensing resistor of the flow speed sensing element to a higher temperature than that of the intake air temperature sensing element by a predetermined temperature difference. Then, the heating current supplied to the heating resistor is regulated in response to the temperature of the heat sensing resistor of the flow speed sensing element, which temperature is changed in response to the quantity of heat transferred to the intake air, so as to maintain the predetermined temperature difference. Consequently, the flow speed of the intake air is measured through the heating current, and the quantity of the intake air is determined by the flow speed.
The above-described intake air quantity measuring device is of a so-called indirect heating type. A device of the so-called self-heating type is also known, as disclosed in Japanese Patent Laid-Open Publication No. 62-177416. Further, there is disclosed a hot film sensor element which incorporates all the resistors including the heating resistor, in pages 105 to 109 of "SAE Technical Paper Series" 880560, published by Society of Automotive Engineers, Inc. in 1988. Therein, it is indicated that the response time of thermal air flow meters, due to a step change of the air flow rate, has two separate phases, i.e., a quick rise with a steep gradient up to about 70% of the final value, followed by a slow, asymtotic creep. In order to avoid this slow approach to the final value causing a significant error, it is proposed to extend the size of the heating resistor beyond that of the sensing resistor to obtain a guard heater effect.
The above-described meter is the device of the indirect heating type with the improved heating resistor, so that the proposed improvement is not applicable to the device of the self-heating type. Further, the improvement is made mechanically to the sensing element in view of the heat transfer therein, so that manufacturing cost of the sensing element will be high to obtain the guard heater effect constantly.